In many communication systems, there is a need to share a communications medium between transmit and receive signals. In e.g. analogue telephony systems, this is known as 4-wire to 2-wire conversion (or vice versa) since the shared medium (twisted pair), has two wires and transmit and receive signals have two wires each.
Typically, there is a need to achieve isolation between transmit and receive paths so that the transmit signal does not reach the receiver input with full strength. This is referred to as near-end echo cancellation. The degree of isolation between transmitter and receiver can be denoted Trans-Hybrid Return Loss (THRL). Here, we define THRL as the ratio between transmitter-receiver loss and the insertion loss from transmitter to line. The functionality can be implemented in different ways, e.g. by using a hybrid coil (also known as a hybrid transformer). Other types of hybrid circuits, e.g. using resistive bridges, are common in e.g. Digital Subscriber Line (DSL) modems. Resistive bridges are cheaper and typically require smaller space on the circuit boards but usually have high loss in at least one direction and they also make it difficult to achieve low noise levels, due to thermal noise in the resistors.
A dual core hybrid coil can be designed to support wide bandwidth but it requires two transformers connected towards the 2-wire side (shared medium). Also, the transformers need to have two separate secondary windings, i.e. a winding with a centre tap is not sufficient. These restrictions lead to high cost and large space requirements, especially if the shared medium has high voltage isolation requirements.
A single core hybrid coil is less complex than the dual core hybrid coil, but it is difficult to achieve high isolation between transmit and receive ports over a wide bandwidth, mainly due to non-ideal properties of the transformer. Some transformer imperfections can be compensated for by modifying a impedance matching device.
For applications with high requirements regarding e.g. common-mode rejection and, it is not always sufficient to use just a transformer as interface between the transceiver and the transmission medium (hereafter called “line”) but a common-mode choke may be needed as well. This is typically seen for Local Area Network (LAN) applications such as 1000BASE-T (gigabit Ethernet) and 10 GBASE-T (10 gigabit Ethernet) where such a choke is used in conjunction with the transformer.
Such devices are available with high bandwidth, good return loss, low insertion loss, and high CMRR. However, even if return loss is very good in normal operation, the choke can cause problems when the device is used as a hybrid coil; if the leakage inductance of the choke is sufficiently high, a negative inductance is needed in the impedance matching device in order to cancel the leakage inductance of the choke, since the leakage inductance of the choke will end up in series with the balance impedance. However, implementing a negative inductance in a one- or two-terminal device requires active components, which typically adds cost, complexity, and noise.